|Increased density of brown trout (Salmo trutta) reduces the size of mottled sculpins (Cottus bairdi) in Black Earth Creek Sections
by Heather Felton
University of Wisconson-Madison
Ecology of Fishes Lab-Zoology 511
A. Brown trout (Salmo trutta) and molted scuplin occupy similar environmental niches.
B. C.R. Ruetz et. al suggest that there is direct competition between slimy sculpins and brown trout.
1. There should be similar competition between mottled sculpin and brown trout in the Black Earth Creek.
2. The experiment done by C.R Ruetz et. al was done in a controlled setting, but also done a stream, so the results should be similar.
C. I looked at the densities of brown trout and mottled sculpin to see if there was a relationship between the density of brown trout and the size of mottled sculpins.
D. This relationship is important because it shows interspecific species competition in this stream habitat between brown trout and mottled sculpin.
1. Brown trout are better competitors. Citation on this?
2. Mottled sculpin are the native species and brown trout are an introduced species.
3. If the relationship exist, it would be yet another example of how introduced species out compete native species for resources.
E. I hypothesize that there will be a liner relationship between the size of mottle sculpins and the density of brown trout, with mottled sculpin size decreasing as the density of brown trout increases. Why linear? Just say a negative relationship.
F. To test this hypothesis, I will compare the densities of brown trout in different reaches of Black Earth Creek and compare them to the size of the mottled sculpin found in the same reach.
II. Methods and Materials -1 for lack of protocol. -0.5 for using wrong statistical test for variables used.
A. Seven sections of two differing areas of Black Earth Creek were used to compose the data set used for this experiment.
1. In each section the fish were caught using electro fishing methods. Protocol for fishing?
2. The length, width, and depth of each section were recorded. Protocol for this? We did transects…how did you go from transect values to section values?
3. Each fish was identified, and the length was taken.
B. Using the information on reach length, width, and depth, I calculated the volume of each section.
1. I excluded the data point for Cross Plains 7, because it was not actually part of the main stream habitat, and was secluded from the rest of the creek.
C. I found the density of brown trout by taking the total number found in each section and dividing it by the number of brown trout caught. You mean BT caught / volume of section, right?
1. In Salmo Cross plains site of Black earth Creek this density might be slightly skewed because catch nets were not set up on the second day of data collection, allowing fish to move down stream.
D. I then found the average size of mottled sculpin for each section, and compared it to the density of brown trout.
E. An ANOVA test was run to check the statistical significance. No. Should be a regression. Two continous variables.
III. Results -0.5 for axes being flipped, given how variables were discussed in the introduction
A. I found that there was a linear relationship between the density of brown trout and the size of mottled sculpin (R2=.3876). and p-values! P-value tells you if the relationship is there or not!
B. As the density of brown trout increased, the average size of mottled sculpin decreased.
1. Thirteen data points were used to establish this trend; residuals, and a normal plot probability were used to check the accuracy of the linear relationship.
2. This relationship was supported by a low p-value of 0.023.
C. A similar trend was seen, but was not as strong when density was calculated by area and not by volume. Should probably flip your axes since you talk about density of brown as the explanatory variable in your introduction.
Figure 1. Mottled sculpin size plotted against the density of brown trout. Each data point represents the average size of mottled sculpin for each of the thirteen sections. There is a linear relationship between mottled sculpin size and the density of brown trout. As brown trout densities increase, mottled sculpin size decreases (p=0.023).
A. My hypothesis that mottled sculpin size would decrease as the density of brown trout increased was supported by the data collected in this experiment.
B. This relationship could exist for several reasons.
1. Both mottled sculpin and brown trout inhabit the same niche, and are in direct competition with brown trout out competing mottled sculpin.
a. This result was found by C. R. Ruetz with slimy sculpin and brown trout.
b. However, conflicting results by J.K.H. Zimmerman were found when studying brown trout and the food web interactions in a different study.
c. These conflicting results might suggest that there are other interactions between the two species that are not understood.
2. Smaller sculpin get enough to eat when living in high density areas of brown trout, but the larger sculpin find areas of the stream with less brown trout to get enough to eat.
a. Another study by J.K.H Zimmerman looks at how trout species interact with slimy sculpin and the consequences on growth.
b. This study could be extrapolated and applied to mottled sculpin.
3. There have been studies done on how sculpin affect the young of brown trout which might have an effect on the respective size and densities of both fish.
a. These interactions are not understood well, and haven’t been studied extensively, but cold be a factor in the trends seen in this system.
C. The reliability of the data seems fairly sound, however, there could be inexactitudes because there was no catch net at the end of each section set up to make sure all the fish were contained.
1. This might skew the data, and a false correlation is seen because fish escaped and the densities are incorrect.
2. The volume in this experiment is also not completely accurate, and could also have affected the trends seen in the data.
D. For further experiments, it would also be useful to look at the densities and size of both fish populations.
1. Both the brown trout and mottled sculpin might be smaller when there are high densities of both.
2. My experimental design did not answer questions about the relation between size and density of both populations of fish, which would be interesting and insightful, and might give greater understanding to the relationships that exist between the two species.
E. The trend seen in this paper, however, is important, and gives important insight into a possible relationship between native and introduced species that has consequences for the success of both species in this stream system.
Clary, J.R. 1972. Predation of Brown Trout by Slimy Sculpin . Progressive Fish Culturist 34: 91.
Hesthagen, T., R. Saksgard, O. Hegge, et al. 2004. Niche overlap between young brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) in a subalpine Norwegian river. Hydrobiologia 521: 117-125.
Hesthagen, T. and J. Heggenes 2003. Competitive habitat displacement of brown trout by Siberian sculpin: the role of size and density. Journal of Fish Biology 62: 222-236.
Pender, D.R. and TJ Kwak 2002. Factors influencing brown trout reproductive success in Ozark tailwater rivers. Transaction of the American Fisheries Society 131: 698-717.
Ruetz, C.R., A.L. Hurford and B. Vondracek 2003. Interspecific interactions between brown trout and slimy sculpin in stream enclosures. Transactions of the American Fisheries Society 132: 611-618.
Zimmerman, J.K.H. and B. Vondracek 2007. Brown trout and food web interactions in a Minnesota stream. Freshwater Biology 52: 123-136.
Zimmerman, J.K.H. and B. Vondracek 2006. Interactions of slimy sculpin (Cottus cognatus) with native and nonnative trout: consequences for growth. Canadian Journal of Fisheries and Aquatic Sciences 63: 1526-1535.